Linkello - Video calls simplified, no installation required.
WhatsApp - WhatsApp Messenger: More than 1 billion people in over 180 countries use WhatsApp to stay in touch with friends and family, anytime and anywhere.
jTorchat - jTorchat stands for Java Torchat, and is a complete rewrite of the popular p2p IM messenger Torchat
Zulip - Chat for distributed teams. Zulip combines the immediacy of real-time chat with an email threading model. With Zulip, you can catch up on important conversations while ignoring irrelevant ones.
Signal - Fast, simple & secure messaging. Privacy that fits in your pocket.
WowApp - Wowapp is the best alternative to Skype and you will get paid to use it!